As very large-scale integrate (VLSI) circuits advance, dimensions of semiconductor devices have been shrinking, and performances of semiconductor devices have been improved accordingly. Due to dimension shrinking of the semiconductor devices, dimensions of interconnects may have to be decreased and consequently may have to carry higher electric current. On the other hand, interconnects are required to have short response time. As a result, conventional interconnect devices are not able to meet the demands on performance of semiconductor devices.
Compared to aluminum (Al), copper has lower resistivity and better resistance to electromigration. Using copper interconnects, resistive-capacitive (RC) delay of interconnects can be reduced. Copper interconnects can also improve electromigration and device reliability of the semiconductor devices. Therefore, copper interconnects are replacing conventional Al interconnects in the semiconductor industry.
However, copper interconnects also have deficiencies. Metallic copper has a high electromigration rate, so copper can diffuse fast in Si, SiO2 and most dielectric materials. Once copper atoms diffuse into semiconductor substrate or dielectric layers, lifetime of minority carriers can be adversely affected and junction leakage current can increase. As a result, circuit failure may occur and reliability of the semiconductor device can be adversely affected.
To solve the problems caused by copper diffusion, a capping layer is often formed on the surface of a metallic layer to prevent copper from diffusing upward into the dielectric layer. Although the capping layer can reduce copper diffusion to a certain extent, performance and reliability of conventional interconnects still need to be improved.